The dynamic performance of a voltage feedback amplifier (i.e., its ability to accurately amplify time-varying signals) is limited by its open-loop unity gain bandwidth or gain-bandwidth product (“GBW”) and slew rate. Because the GBW of an amplifier is constant, the closed-loop bandwidth is inversely proportional to its gain; if a given amplifier is configured for a high gain, for example, its bandwidth correspondingly drops (sometimes dramatically). This drop in bandwidth may degrade the amplifier's performance, especially with high-frequency inputs. Some amplifiers (such as general-purpose instrumentation amplifiers) include an input ports (e.g., pins on a chip package) to which a customizable gain-setting resistor (“RG”) may be connected. A user may therefore select a desired gain value by varying the size of RG but, in doing so, deleteriously change the amplifier's bandwidth. FIG. 1 illustrates an exemplary three operational-amplifier (“op-amp”) instrumentation amplifier 100 that includes input amplifiers 102 (divided into first-stage A1 and second-stage A2 amplifiers) and an output amplifier 104. Because the output amplifier 104 is configured to have a gain of one, its closed-loop bandwidth is fixed at approximately half of its unit-gain bandwidth, regardless of the gain settings of the entire amplifier 100. Therefore, the bandwidth of the amplifier 100 is usually limited by the bandwidth of the input amplifier 102. For example, assuming the input amplifier 102 has a unit-gain bandwidth of 100 kHz, at a gain of 1000, the closed-loop 3 dB bandwidth (i.e., cutoff frequency) of the amplifier 100 is reduced to 100 Hz (because, as noted above, its GBW is fixed, and 100 kHz÷1000=100 Hz).
If the gain is set to a high value, the resulting low bandwidth of the amplifier 100 may be increased by configuring other components in the circuit. For example, part of the amplifier's compensation capacitance 106 may be switched out (i.e., electrically disconnected from the circuit) to increase its open loop unity gain bandwidth (and, as a result, its closed loop bandwidth as well) thereby improving the high-frequency performance of the circuit. This adjustment, however, requires additional input ports for the control signals necessary to change the compensation capacitance 106; in the simplest case, one pin may be used to switch part of the capacitance 106 in or out, but more pins are required for finer-grained control. Many amplifiers, such as commercial general-purpose resistor-programmable instrumentation amplifiers, cannot provide these additional ports because they would increase the cost of the amplifier, the complexity of the control circuitry, and/or the size of the amplifier package. A current-feedback amplifier may be used for high-speed operation, because its dynamic performance is not limited by GBW and slew rate, but these amplifiers have lower DC gain and are thus not suitable for high-precision applications. A need therefore exists for a way of boosting the bandwidth of a voltage-feedback amplifier at high gains without requiring additional input ports.